1. Field of the Invention
The present invention relates to optical fibers. More specifically, the present invention relates to techniques for testing the bending stress of optical fibers.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
Certain fiber optic applications place significant demands on the tensile strength of the optical fiber. One such stressful application is in the deployment of fiber during the flight of a fiber optic guided missile. During high speed payout of optical fiber from a bobbin, the fiber is subjected to high stress at the peel point. The fiber is bent to a small radius at this peel point. The higher the velocity, the smaller the bend radius and the higher the stress. It is estimated that this bend radius is approximately 0.060" at a payout velocity of 700 feet per second.
Therefore, certain tensile and bending tests have been developed to detect flaws, nicks, cracks and other bad spots on the buffer of the fiber which would cause a fiber to break under such stress. (The tensile test is a proof test which measures the amount of tension the fiber can withstand. The bending test provides a measure of the extent to which the fiber may be bent under pressure before breakage or other damage occurs.)
One such test involves the simple application of tension to the fiber which approximates, to some degree, the stress the fiber will experience during payout. However, this test provides no measure of the bending strength of the fiber. In addition, the time at which the fiber is at high stress is too long, that is, such prolonged stress testing is known in the art cause cracks to grow, to degrade the strength of the fiber (by a phenomena known as static fatigue) and to otherwise pose the hazard of considerable and unacceptable damage to the fiber.
Another test involves the use of a mandrel. A mandrel is essentially a round shaft or pin on a bearing. The fiber is bent around the shaft or pin to provide a measure of the bending and tensile strength of the fiber. Unfortunately, this test is generally capable of testing the bending stress of the fiber in a single plane. That is, mandrel tests are not capable of testing all planes around the periphery of the fiber.
Thus, there is a need in the art for an inexpensive method and apparatus for providing high speed, nondestructive bend testing of the entire periphery of an optical fiber along its entire length.